1. Field of the Invention
The present invention relates to a hard disk drive calibration method and apparatus, and more particularly, to a method of calibrating parameters used to measure a back electromotive force (BEMF) of a voice coil motor (VCM), a method for measuring a BEMF of a voice coil motor, and an apparatus therefore.
2. Description of Related Art
In order to accurately control a seek operation of a hard disk drive, it is necessary to detect a velocity of a head. The velocity of the head is calculated by using servo information recorded on a surface of a disk of the hard disk drive. More particularly, the velocity of the head is calculated by using track numbers and sector numbers included in the servo information.
The method is applicable only if the head is able to read the servo information. For example, in a ramp loading/unloading type hard disk drive, the head is not located on the disk before the head is loaded on the disk or after the head is unloaded from the disk since the servo information recorded on the disk cannot be read, it is impossible to calculate the velocity of the head.
For this reason, the ramp loading/unloading type hard disk drive measures the velocity of the head by a back electromotive force (BEMF) of a voice coil motor (VCM) which drives a head slider. That method is based on the fact that the BEMF is proportional to a rotational velocity of the VCM, i.e., the velocity of the head.
Recently, even in hard disk drives that do not use the ramp loading/unloading method, the velocity of the head is measured by the BEMF of the VCM. In such a hard disk drive, the measured velocity of the head is fed back to control the velocity of the head during an unlatch operation. In addition, even in a seek servo operation, the velocity of the head is measured by using the BEMF of the VCM, so that the velocity of the head can be controlled in acceleration or deceleration regions.
FIGS. 1A and 1B are views schematically showing operations of loading/unloading a head slider in accordance with a ramp loading method, respectively. A hard disk drive using the ramp loading method comprises a ramp 6 and a protrusion member 3b provided at the end of an actuator 3. As a VCM driving current is applied to a VCM 5, the actuator 3 rotates around a driving shaft 3a, so that the protrusion member 3b can slide on surface regions 6a to 6d of the ramp 6.
In FIG. 1A, the counter clockwise and clockwise rotations of a head slider 4 correspond to loading and unloading directions, respectively. After the head slider 4 is unloaded from a disk 1, the actuator 3 ends up in contact with the surface region 6d, which is a head-parking surface of the ramp 6. At this time, a coil support member 3c is in contact with or close to an outer crash stop (OCS) 7.
In a process for loading the head slider 4, the actuator 3 rotates in the loading direction to move the head slider 4 on the disk 1. The protrusion member 3b sequentially slides across the surface regions 6a to 6c of the ramp 6 and reaches the surface region 6d. The surface region 6a is a slanted region, as shown in FIG. 1B.
In a process for unloading the head slider 4, the actuator rotates in the unloading direction to move the head slider 4 to the surface region 6a, that is, the head-parking region. More specifically, the protrusion member 3b sequentially slides across the surface regions 6d to 6b and reaches the surface region 6a. The head connected to the actuator 3 is parked on the surface region 6a, as shown in FIG. 1B.
As shown in FIGS. 1A and 1B, when the head slider 4 enters or leaves the area over the disk 1 by the loading or unloading process, it is necessary to control the velocity of the head slider 4 in order to prevent collision between the head slider 4 and ramp 6 or scratches on the disk 1. For this reason, the velocity of the head slider 4 is detected. In addition, in the unloading process, the actuator 3 may collide with the OCS 7. In order to reduce the impact and associated noise, it is necessary to detect and control the velocity of the head slider 4.
As shown in FIG. 1A, while the head slider 4 is being loaded on the disk or being unloaded from the disk, the head slider 4 does not exist on the disk 1 but exists on the ramp 6.
Therefore, the servo information recorded on the disk 1 cannot be read, and it is impossible to calculate the velocity of the head. For this reason, the velocity of the head is measured by the BEMF of the VCM 5.
FIG. 2 is a block diagram showing a VCM control apparatus. The VCM control apparatus includes: a VCM driving unit 202, a sensing resistor 206 having a sensing resistance Rs, a BEMF detection unit 208, an analog-to-digital converter (ADC) 210, a microcontroller 212, and a digital-to-analog converter (DAC) 214. The VCM driving unit 202 drives the VCM 204. In FIG. 2, the VCM 204 is represented by a coil resistance Rm and a coil inductance Lm of itself in a form of an equivalent circuit. The sensing resistor (Rs) 206 senses a current through the VCM 204. The BEMF detection unit 208 detects the BEMF induced in the VCM 204 by a VCM voltage Vvcm applied to the VCM 204 and a VCM current Im sensed by the sensing resistor (Rs) 206. The ADC 210 converts an analog output of the BEMF detection unit 208 into a digital signal. The microcontroller 212 controls the VCM 204. The DAC 214 converts a digital VCM control signal applied by the microcontroller 212 into an analog VCM control signal and applies the analog VCM control signal to the VCM driving unit 202.
The BEMF detection unit 208 includes: a VCM voltage amplifier 208a for amplifying a VCM voltage applied to the VCM 204; a VCM current amplifier 208b for amplifying a VCM current Im detected by the sensing resistor (Rs) 206; and a differential amplifier 208c for amplifying a difference between outputs of the VCM voltage and current amplifiers 208a and 208b. 
The VCM voltage amplifier 208a, the VCM current amplifier 208b, and the differential amplifiers 208c have their own gains Ga, Gb, and Gt, respectively. The gain Ga is substantially 1. The VCM voltage amplifier 208a converts a VCM voltage into an amplified voltage, so called voltage-to-voltage amplifier, while the VCM current amplifier 208b converts a VCM current into an amplified voltage using the sensing resister Rs, so called current-to-voltage amplifier.
Referring to FIG. 2, since the VCM current Im flows through the VCM 204, the VCM voltage Vvcm is represented by the following equation:Vvcm=Lm×dlm/dt+Rm×Im+Vbemf.
A voltage Vs across the sensing resistor Rs 206 is represented by the following equation:Vs=Rs×Im.
Here, dim/dt is the time derivative of the VCM current Im. Vbemf is a voltage induced by the BEMF of the VCM and is proportional to a rotational velocity of the VCM, that is, the velocity of the head. Since a BEMF constant of the VCM is a known value, it is possible to calculate the rotational velocity of the VCM, that is, the velocity of the head by using the BEMF voltage Vbemf and the BEMF constant.
The VCM voltage Vvcm is amplified by the VCM voltage amplifier 208a with its gain Ga of 1. The voltage Vs across the sensing resistor 206 is amplified by the VCM current amplifier 208b with its gain Gb. The difference between the outputs of the amplifiers 208a and 208b is amplified by the differential amplifier 208c with its gain Gt. The output of the differential amplifier 208c is applied to the ADC 210. The voltage Vadc measured by the ADC 210 is a differential voltage, that is, a difference between one voltage across the coil resistor Rm and the coil inductance Lm and the other voltage across the sensing resistor Rs 206, represented as follows:
                    Vadc        =                ⁢                  Gt          ×                      (                          Vvcm              -                              Gb                ×                Vs                                      )                                                  =                ⁢                  Gt          ×                                    (                                                Lm                  ×                                                            ⅆ                      Im                                        /                                          ⅆ                      t                                                                      +                                  Rm                  ×                  Im                                +                Vbemf                -                                  Gb                  ⁢                                                                          ⁢                                      x                    ⁢                                          [                      *                                        ]                                    ⁢                                                                          ⁢                  Rs                  ×                  Im                                            )                        .                              
Since the amplifiers 208a to 208c have their own offsets, the voltage Vadc measured by the ADC 210 also has its own offset voltage Voffs.
In the VCM control unit shown in FIG. 2, the gain Gb of the VCM current amplifier 208b can be adjusted by the microcontroller 212.
Assuming that the VCM current Im applied by the VCM driving unit 202 is constant, the voltage Vadc measured by the ADC 210 is obtained by applying the offset voltage Voffs. The voltage Vadc is represented by Equation 1:Vadc=Gt×{(Rm−Gb×Rs)×Im+Vbemf}+Voffs.  [Equation 1]
If the gain Gb of the VCM current amplifier 208b can be set by using the ratio Rm/Rs of the coil resistance Rm to the sensing resistance Rs, the voltage Vadc measured by the ADC 210 is given by:Vadc=Gt×{(Rm−Rm/Rs×Rs)×Im+Vbemf}+Voffs=Gt×Vbemf+Voffs
Therefore, if the coil resistance Rm and the offset voltage Voffs are known, the BEMF voltage Vbemf proportional to the velocity of the head, that is, the velocity of the VCM 204, can be calculated.
In the conventional art, the ratio Rm/Rs and the offset voltage Voffs are calibrated by using the crash stop just before the head is loaded on the ramp.
For example, if a current of 0 mA is applied to the VCM 204 and a head slider is being parked on the ramp 6, the voltage Vadc measured by the ADC 210 is equal to the offset voltage Voffs. In the hard disk drive comprising the ramp loading/unloading type latch system, the protrusion member 3b or a latch device using a magnetic force is disposed at the location corresponding to the parking position of the head. When the current of 0 mA is applied to the VCM 204, the VCM 204 does not move. As a result, the BEMF voltage Vbemf is 0 V, so that it is possible to measure the offset voltage Voffs.
Next, the VCM 204 is driven, so that a force can be exerted towards the OCS. With respect to various values of the gains Gb, it is determined which value of the gain Gb corresponds to the voltage Vadc measured by the ADC 210 closest to the offset voltage Voffs. Since the VCM 204 stops due to the OCS, the BEMF voltage Vbemf is 0 V. The voltage Vadc measured by the ADC 210 is given by:Vadc=Gt×(Gm−Gb×Rs)×Im+Voffs.
Here, as the gain Gb of the VCM current amplifier 208b converges to the ratio Rm/Rs, the voltage Vadc measured by the ADC 210 approaches the offset voltage Voffs.
In order to accurately measure the BEMF voltage Vbemf by using the apparatus shown in FIG. 1, the offset voltage Voffs and the gain Gb of the VCM current amplifier 208b have to be calibrated.
The gain Gb of the VCM current amplifier 208b is a binary value adjusted by a microcontroller 212. Therefore, even though a DAC converter (not shown) of the microcontroller 212 to output the gain Gb of VCM current amplifier 208b has a high resolution, there is essentially a quantization error in the gain Gb. As a result, even if the gain Gb of the VCM current amplifier 208b is well adjusted, a value of Rm−Gb×Rs may not be 0 due to the quantization error.
Hereinafter, the value of Rm−Gb×Rs is referred to as a “slope S.” The voltage Vadc measured by the ADC 210 is represented by Equation 2.Vadc=Gt×(S×Im+Vbemf)+Voffs  [Equation 2]
The slope S can be corrected by using the crash stop. If the gain Gt of the differential amplifier 208c is known and the offset voltage Voffs and the gain Gb of the VCM current amplifier 208b are calibrated, the VCM 204 is driven toward the crash stop. At the same time, two different levels of current are made to flow through the VCM coil. The slope S can be corrected by using the voltage Vadc measured by the ADC 210 in the two cases.
As described above, in order to measure the BEMF of the VCM, it is necessary to calibrate the offset voltage Voffs, the gain Gb of the VCM current amplifier, and the slope S induced from the finite resolution of the gain Gb. Here, the word “finite resolution” means that its precision is limited by a number of bits to represent certain value.
Before the head is loaded, the offset voltage Voffs, the gain Gb of the VCM current amplifier 208b, and the slope S can be calibrated by using the crash stop described above.
The ratio Rm/Rs of the coil resistance Rm of the VCM 204 to the sensing resistance Rs used to set the gain Gb of the VCM current amplifier 208b is sensitive to a temperature of the VCM coil. In particular, the coil resistance Rm of the VCM 204 is changed dramatically according to the temperature.
Although the offset voltage Voffs, the gain Gb of the VCM current amplifier 208b, and the slope S are set before the head is loaded, the temperature of the VCM coil may change due to various causes. As a result, since the coil resistance Rm of the VCM 204 changes, it is necessary to reset the gain Gb of the VCM current amplifier 208b and the slope S.
In this case, if the change in the temperature of the VCM coil is obtained, the gain Gb of the VCM current amplifier 208b and the slope S can be reset. However, an additional temperature sensor is needed, which is disadvantageous. In addition, it is very difficult to measure the temperature of only the VCM coil.
Many types of hard disk drives include temperature sensors for measuring their own operating temperatures. However, since the temperature of the VCM coil changes with the current through the VCM coil, the gain Gb of the VCM current amplifier 208b and the slope S reset based on the operational temperature of the hard disk drive are very inaccurate.
Meanwhile, there is a method of resetting the gain Gb of the VCM current amplifier 208b and the slope S by using an inner crash stop (ICS) similar to the aforementioned calibration method performed just before the head is loaded. However, in the method, the head slider 4 must impact on the ICS, which may lead to an undesirable result. In addition, after the head moves to an inner diameter area of the disk 1 to reset the gain Gb of the VCM current amplifier 208b and the slope S, the head is unloaded. Therefore, it takes a long time to perform the unloading operation.
In order to solve the problems of the conventional art, a method of correcting the slope S before the head is unloaded and a method of resetting only the gain Gb of the VCM current amplifier 208b before the head is unloaded have been proposed. The method of correcting the slope S is disclosed in U.S. Pat. No. 6,229,663. In the method of resetting only the gain Gb of the VCM current amplifier 208b without taking the slope S into consideration, while a seek operation is performed to move the head between two positions of the disk, the voltages Vadc are measured with the ADC 210. Next, the gain Gb of the VCM current amplifier 208b is obtained so that the average of the voltages Vadc can be equal to the offset voltage Voffs.
In the conventional calibration method, since repetitive seek operations are performed to correct the gain Gb of the VCM current amplifier 208b and the slope S before the head is unloaded, it takes long time to perform the calibration. In particular, in the method of measuring the gain Gb of the VCM current amplifier 208b without taking the slope S into consideration, the measured BEMF voltage Vbemf may be very inaccurate.
If the gain Gb of the VCM current amplifier 208b and the slope S are not accurately corrected, the measured BEMF voltage Vbemf of VCM 204 is so inaccurate that the unloading operation cannot be controlled properly.